1. Field of the Invention
The present invention relates to a voltage non-linear resistor comprising zinc oxide as a principal ingredient, and more particularly to a voltage non-linear resistor which is excellent in life expectancy under electrical stress, current impulse withstand capability, discharge voltage ratio, change rate of discharge voltage after applying current impulse and water penetrating characteristics.
2. Description of the Prior Art
Heretofore, there have been widely known resistors comprising zinc oxide as a principal ingredient and small amounts of additives, which exhibit an excellent voltage non-linear characteristic. Utilizing such a characteristic, these resistors have been used in, for example, lightning arresters and the like.
In particular, when they are used as a lightning arrester, even if an excessive current flows by a lightning strike, the current is grounded by the voltage non-linear resistor which is usually an insulator and turns to a conductor when a voltage exceeds a preestimated level. Thus, accidents due to lightning strikes can be prevented.
There have hitherto been disclosed Bi, Co, Mn, Sb, Cr, Si, Ni, Al, B, Ag and Zr as an applicable additive, for example, in Japanese Patent Application Publication No. 59-41,285 and Japanese Patent Laid-open Application Nos. 62-237,703, 63-136,603 and 1-228,105.
Meanwhile, many have expected the develop of a voltage non-linear resistor that is excellent in all electrical characteristics to be provided by voltage non-linear resistors, such as long life under electrical stress, current impulse withstand capability, discharge voltage ratio, change rate of discharge voltage after applying current impulse and water penetrating characteristics. Although each characteristic is good according to the techniques disclosed in the each of the above patent applications, difficulties have been encountered in satisfying all the above 5 particulars.
Resistors are required to have a long life under electrical stress to be stabilized for a long period of time without thermal runaway, being induced by an applied voltage. Namely, with respect to the life under electrical and thermal stresses converted from an Arrhenius' plot, the resistors are desired to have a good performance for at least 50 years, preferably at least 100 years under a voltage applying rate of 85% at 40.degree. C.
Further, the resistors are required to have a current impulse withstand capability high enough to withstand fracture due to current impulse. Namely, a lightning current impulse withstand capability which is determined as an energy value (passed value) converted from a withstand capability after 2 repetitions, with a 5 minute interval, of applying lightning current impulse with a waveform of 4/10 .mu.s is desired to be at least 16 KJ. The switching current impulse withstand capability which is determined as an energy value (passed value) converted from a withstand capability after 20 repetitions of applying switching current impulse with a waveform of 2 ms is desired to be at least 16 KJ.
On the other hand, the discharge voltage increases with decreasing voltage non-linearity, in a large current region. Accordingly, it is required that the voltage non-linearity is high, namely, the discharge voltage is low, even in the large current region. Namely, the discharge voltage ratio which is defined as a ratio of a varistor voltage (discharge voltage at a 1 A current: hereinafter referred to as "V.sub.1A ") to a discharge voltage, for example, at a 40 KA current (V.sub.40KA) is desired to be less than 2.0.
Further, the resistors are required to have voltage-current characteristics hardly deteriorated due to current impulse, i.e., a low change rate of discharge voltage after applying current impulse. For example, change rate of varistor voltage (.DELTA.V.sub.1A) before and after 10 repetitions of applying current impulse of 40 KA with a waveform of 4/10 .mu.s is desired to be within 5%.
Furthermore, as for water penetrability, there is seen a phenomenon such that water permeates through micro-cracks or the like into a resistor. The water penetrability is evaluated by a fluorescent flaw detective test described hereinafter. With regard to a water penetrative resistor, deterioration of characteristics of the resistor is not recognized under dry conditions. However, the life under electrical stress and the current impulse withstand capability deteriorate under moisturized conditions. Therefore, water penetrating characteristics are important in respect of a long-term reliability. Particularly, the water penetrating characteristics are important to resistors to be applied to lightning arresters or the like to be used outdoors.
Thus, voltage nonlinear resistors to be used as a lightning arrester or the like are required to satisfy simultaneously the above-described 5 characteristics. Particularly, in order to make a resistor compact (by decreasing its length), the varistor voltage of the resistor should be increased while the discharge voltage ratio is kept low. Namely, in the case of a small-sized lightning arrester designed as a resistor having a high varistor voltage (V.sub.1mA .ltoreq.300 V/mm), the above-described lightning current impulse withstand capability is desirably at least 13 KJ and the switching current impulse withstand capability is desirably at least 11 KJ. Further, the discharge voltage ratio which is defined as a ratio of a varistor voltage at a 1 mA current (V.sub.1mA) to a discharge voltage, for example, at a 30 KA current (V.sub.30KA) is desired to be less than 2.2. Furthermore, the change rate of varistor voltage (.DELTA.V.sub.1mA) before and after 10 repetitions of applying current impulse of 40 KA with a waveform of 4/10 .mu.s is desired to be within 10%. However, resistors having a high varistor voltage such as V.sub.1mA .ltoreq.300 V/mm which can satisfy all the above 5 particulars have not yet been obtained.